Particle Gravimetric Analysis

Gravimetric analysis in fluids requires less skill and equipment than microscopic particle counting. Once the specification has been established by weight, the gravimetric method provides a simple, inexpensive and highly reproducible routine control measure. For Particle Contamination in Petroleum Products (D2274) and Aviation Fuels (D2276) the ASTM recommends a gravimetric and color rating technique.

Introduction

Gravimetric analysis involves filtering a contaminated sample through a control filter and a sample filter. In this method, you place two preweighed filters, one on top of the other, in a single filter holder then filter a sample. Particulate contaminants will be retained entirely by the top test filter. However, both filters are subjected to identical alterations in tare weight as a result of moisture loss or gain, sample adsorption or desorption, and other environmental factors. Any change in weight of the bottom (“control”) filter is then applied as a correction to the weight of contaminant. The contaminant weight is determined by reweighing the test filter and subtracting its original tare weight. Results accurate to 0.1 mg are routinely attained using this method.

Filter Selection

The simplest gravimetric analyses use matched weight cassettes. Each cassette contains two MilliporeSigma filters that are matched in tare weight to 0.1 mg. These cassettes are factory-assembled so that preweighing each membrane in the field before filtering the sample is unnecessary. After sampling, the weight of the contaminant is determined simply as the difference in weight between the two membranes.

Matched-weight cellulose filters (type AA), 0.8 µm pore size, are preweighed to within 0.1mg. These are available in 47 mm discs, 50 pairs per package, and in 37 mm matched-weight cassettes. Call Technical Service for more details, or visit:

Sample Preparation for Gravimetric Analysis

The first three steps may be omitted when testing samples from air and other gases, water and wholly volatile solvents. All steps must be followed with viscous liquids such as paints, hydraulic oil, and turbine fuels.

Sample Preparation for Gravimetric Analysis

Insert the aerosol adapter into stopper on the vacuum flask.

Placing aerosol adapter into rubber stopper, hose end down.

Remove plugs from cassette and mount the cassette, filter side up, on the aerosol adapter.

Cassette containing sample is fitted to Luer slip of adapter, and stopper is fitted into filter flask (inlet plug removed).

Apply vacuum and introduce membrane-filtered solvent through the top opening using a solvent dispenser. Release vacuum.

Open the cassette and transfer filters into covered glass petri dishes.

Loosen the lids of the glass petri dishes and place in an oven at 90 °C for 30 minutes.

Remove the dishes from the oven. With lids ajar, allow the filter to cool and equilibrate to ambient conditions for at least 15 minutes.

Introducing flushing solvent through top opening of cassette using solvent filtering dispenser.

Weighing and Calculation

The procedure for calculating the results of your gravimetric analysis samples depends on the filter method used during sample collection.

Matched-Weight Filters or Matched-Weight Cassettes

Reweigh both filters and record the weights.

Subtract the weight of the control filter from the weight of the test filter. The test filter will normally be heavier than the control filter. Negative results should be recorded as “zero” contamination.

Typical results would be:

Test #

1

2

3

Final weight of test filter (mg)

49.20

51.30

50.80

Final weight of control filter (mg)

48.50

50.70

50.35

Results in:

mg/volume filtered

0.70

0.60

0.45

Control Filter Method

Reweigh the filters and record the final weights.

Subtract the initial weight from the final weight of each test filter.

Determine the loss or gain in tare weight of the control filter by appropriate subtraction. A weight increase greater than 0.5 mg in the control filter indicates inadequate flushing of residual test fluid from the filter. The test should be rerun.

Apply the control filter weight change as a correction factor to the test result.

Typical results would be:

Test #

1

2

3

Control

Final weight (mg)

49.20

51.30

50.80

49.40

Initial weight (mg)

48.00

49.95

49.65

49.10

Weight (mg)

1.20

1.35

1.15

0.30

Control factor

-0.30

-0.30

-0.30

-0.30

Results in:

mg/volume filtered

0.90

1.05

0.85

Inorganic (Noncombustible) FractionThe inorganic fraction of the particle weight is easily determined by ashing the filter. This procedure can be used with filters made of mixed cellulose esters or other ashable material.

Clean and ignite a small porcelain crucible.

Place in a muffle furnace at 750°C for 20 minutes.

Allow the crucible to cool in a desiccator and weigh it to the nearest 0.05 mg.

Repeat steps 2 and 3 until the crucible has constant weight.

Place the filter containing the contaminant residue in the crucible. Wet it with ethanol and carefully ignite the filter.

Cover the crucible and place it in the muffle furnace at 750°C for 20 minutes.

Allow the crucible to cool in a desiccator and reweigh it. As the organic sediment will have been ignited, the final weight difference represents the inorganic particle contamination